Electroplating baths and method for the electrodeposition of zinc



Patented Aug. 23, 1949 ELECTRGPLATING BATES AND METHOD FOR THEELEGTRODEPOSITIUN OF ZINC Allan E. Chester, Highland Park, and FrederickF. Beisinger, Waukegan, Ill., assignors to Poor & Company, Chicago,151., a corporation of Delaware No Drawing. Application March 20, 1944,Serial No. 527,360

6 Claims.

This invention relates to new and improved cyanide-zinc plating baths,and to a new and improved method for the electrodeposition of zinc.

One of the objects of the invention is to produce new and improvedcyanide-zinc plating baths.

A further object is the preparation of new cyanide-zinc electroplatingbaths useful over a wide range of current densities without burning theelectroplated material.

Another object of the invention is to provide new and improved alkalinezinc cyanide electroplating baths which are effective in producingsemi-bright, exceptionally smooth zinc plated articles having a pleasinguniform color, while using lower ratios of total sodium cyanide to zincmetal (M ratios) than has heretofore been considered possible for theproduction of brightening effects in the electrodeposition of zinc.

Another object of the invention is the provision of a new and improvedmethod for the electrodeposition of zinc. Other objects will appearhereinafter.

In the electrodeposition of zinc from alkaline cyanide electrolytes inaccordance with the invention, it has now been found that theincorporation of a chromic aldonate into a cyanide zinc plating bathmakes it possible to electrodeposit zinc over a Wide range of currentdensities without burning the plated material. Furthermore, by employingcyanide-zinc plating baths of the type herein described, semi-bright,very smooth zinc plated finishes having a pleasing uniform color areobtained with lower M ratios (ratio of total sodium cyanide to zincmetal) than heretoiore used in obtaining brightening effects.Additionally, cyanide-zinc plating baths contain ing chromic aldonatehave enhanced throwing and coloring power.

The term chromic aldonate is used herein to describe compounds oftrivalent chromium with aldonic acids which may also contain alkalimetals, or the like, in their molecules. These compounds are usuallyprepared in solution form and are preferably made by reacting thesolution of the aldonic acid with an alkaline solution of a chromatesuch as may be prepared, for example, by dissolving chromic anhydride(CrOa) in water and adding sodium hydroxide. They can also be preparedby reacting the dichromates with an al.

2 donic acid. In our co-pending application Serial No. 527,359, filed ofeven date herewith, which has matured into United States Patent No.2,428,356, we describe the chromic aldonates as the product of thereaction of an aqueous alkaline solution of a chromium compound selectedfrom the group 01 trivalent and hexavalent chromium compounds with analdonic acid, the ratio of the aldonic acid molecule to the chromiumatom being at least 1:1. The chromic aldonate solutions in concentratedform are colored a deep green. When added in relatively small amounts tocyanide-zinc plating baths they impart thereto a brilliant clear greencolor. These baths are stable and do not tend to precipitate ordeteriorate on standing.

The invention will be further illustrated, but is not limited, by thefollowing examples:

Example I This example illustrates the preparation of a chromicgluconate.

An aqueous solution was prepared b mixing together 1100 grams (11 moles)of chromic acid (ClOs) and 1650 cc. of water. 900 grams (22.5 moles) ofsodium hydroxide were then dissolved in this solution which became warm.This produced an alkaline approximately 21-22% solution of sodiumchromate (11 moles). Thereafter, 6000 cc. of 50% gluconic acid (about 15moles of gluconic acid) containing about 1 to 4% dextrose or otherreducing sugars was added with agitation. In 4 or 5 minutes theresultant solution became very hot, with foaming and boiling and theliberation of carbon dioxide. At the end of 1 to 2 hours the foamsubsided, the product obtained being a, very deep green solution. Thissolution exhibited no precipitation or co-agulation on standing, and maybe diluted with water to a beautiful green colored solution. Thespecific gravity of the concentrated solution was approximately 1.322,

Example II agitation. A green colored product was obtained as in ExampleI, having similar properties.

Example III This example illustrates the preparation of alkaline zinccyanide electrolytes utilizing chromic gluconates as prepared inExamples I or II.

An electrolyte was prepared by mixing together the followin ingredients:

95-100 grams sodium cyanide 45-55 grams zinc metal (added in the form ofzinc oxide) 100-120 grams sodium hydroxide and enough Water to make 1liter of solution 2 grams per liter of zinc dust was stirred into theelectrolyte, and the entire mixture was filtered in order to removetraces of heavy metals.

This electroplating bath, when employed in a conventional manner,ordinarily produces zinc electroplated articles with a dull gray Uponthe addition of either of the products of Examples I or II, however,preferably in amounts corresponding to from to 60 cc. of either of saidproducts per gallon of the above described alkaline zinc cyanideelectrolyte, semi-bright, -very smooth finishes, having a pleasing,uniform color, were obtained. Furthermore, it was found that the complexaldonate compositions of Examples I or II had a remarkable anti-burneffect, and made it possible to obtainuniform, light gray colcred zincplates over a-wide current density range (-25-250' amperes per squarefoot).

One of the important features of the invention is the discovery thatwith such electrolytes substantial brightening effects are obtained withM ratios within the range of 1.75 to '2'.2. The expression "Mratioj aspreviously indicated, is the ratio of the total weight of sodium cyanidethebath to the total weight of zinc metal. Heretofore it had beenconsideredthat an M ratio 2.25 to 1 was the lowest. M ratio at whichbrightening effects in zinc plates could be obtained. See: Transactionsof Amer can Electrochemical Society, volume 80, page 416 (1941). It iswell lg-nownthat higher-M ratios decrease the cathode efiei ncy. H w e yth ncorporation of a ehromic gluconate into a cyanide zincelectroplating bath,, as heretofore describeman M ratio as-lowas 1.75 to1 can be used and the cathode eificiencycan be increased to ii-996%. Atthe same time, high current densities, preferably up to 120 amperes persquare foot, which is about all that is practical with presentcommercial installations, Canbe employed without burning the-zinc platedarticle; By using alternating currents superimposed upon direct currentsat good pattern between high current and low current areas can beobtained. Excellent results are ob tained with current densities around70-80 ampares per square foot; As an illustration of. the eifieiency ofthe plating operation, at 80 amperes perv square foot, it is possibletoplate0.000l" of zinc per minute with a plating bath of the typedescribed in Example III.

The invention is susceptible to some variation and modification in themanner of its practical application. In general, of the chromicaldonates it, is preferable to employ a chromic" gluconate because thegluconates are the most cheaply and readily available of all of thechromic'aldonates. Gluoonic acid is ordinarily employed'in thepreparation of the chromic gluconates in-the form of aqueoussolutionwhich usually hasa concentration not substantially greater than 50%because at the higher concentrations it tends to go into the crystallinelactone form. As examples of other aldonic acids which may be used toprepare chromic aldonates, mention is made of the following: mannonic,arabonic, galactonic, and xylonic. The aldonic acids are obtained by theoxidation of the corresponding aldoses and all of them, includinggluconic acid, have alpha and beta lactone forms.

The invention-is hereby claimed as follows:

1. An aqueous cyanide-zinc electroplating bath comprising an alkalinecyanide-zinc solution having an M ratio within the range of 1.75 to 2.2and containing an effective quantity of the prodnot of the reaction ofan aqueous alkaline solution of a chromium compound selected from thegroup consisting .of trivalent and hexavalent chromium compounds and analdonic acid, the ratio of the ald'onic acid molecule to the chromiumatom in said reaction product being at least 1:1, said product existingin the form of clear green stable solutions in water.

2. An aqueous cyanide-zinc electroplating bath comprising an alkalinecyanide-zinc solution having an M ratio Within the range of 1.75 to 2.2and containing an effective quantity or the product of the reaction ofan aqueous alkaline solution of a chromium compound selected from thegroup consisting of trivalent and heXaVa-le'nt chromium compounds and agluconic acid, the ratio of thegluconic acid molecule to the chrorniumatom in said reaction product being at least Izl, said product existingin the form of clear green stable solutions in water.

3. In a method of electrodepositing zinc the step which compriseselectrodepos-itingthe zinc from an alkaline aqueous cyanide-zinc platingbath having an M ratio within the-range of 1.75 to 2.2 to which has beenadded an effective qua-n tity of the product of the reaction of anaqueous alkaline solution of a chromium compound selected' from thegroup consisting of trivalent hexavalent chromium compounds and an'aldonic acid, the ratio of the aldonic acid molecule to the chromiumatom in said reaction product being at least 121, said product existingin the form of clear green stable solutions in water.

4. In a method of electrodepo'sitin'g zinc step which compriseselectrodepositing the Zinc from an aikaline aqueous cyanide-zinc platingbath having an M ratio within the range of 1.75 to 2.2 to which has beenadded anefiec'tive quantity of the product'of the reaction of an aqueousalkaline solution of a chromium compound selected from the groupconsisting of trivalent and hexavalent' chromium compounds and agluconic acid, the ratio of the gluconic acid molecule to the chromiumatom in said reaction product being at least 1:1', said product existingin the form of clear green stable solutions in water,

5'. A method of electrodepositing Zinc which comprises electrodepositingthe zinc from an alkaline aqueous cyanide-zinc plating bath having an Mratio within the range of 1.75 to 2.2 to which has been added 30 to 60cc. per gallon of bath of the product of the reaction of anapproximately 50% aqueous solution of gluconic acidcontaining less than4% of reducing sugars and an aqueous-alkaline solution of a chromiumcompound selected'from the group consisting of trivalent and hexavalentchromium compounds, the ratio of the gluconic acid molecule to thechromium atoms being at least 1-:1, said product enistingin the'formofclear green stable solutions in Water, and effecting saidelectrodeposition at current densities within the range of 25 to 250amperes per square foot.

6. An alkaline aqueous zinc-cyanide electroplating bath having an Mratio Within the range of 1.75 to 2.2 and comprising about 30 to 60 cc.per gallon of bath of the product of the reaction of an approximately50% solution of gluconic acid containing about 1 to 4% of reducingsugars with an alkaline approximately 21 to 22% solution of sodiumchromate, the relative proportions of sodium chromate to gluconic acidbeing approximately 11 mols of sodium chromate to 15 mols of gluconicacid, said product existing in the form of clear green stable solutionsin water,

ALLAN E. CHESTER. FREDERICK F. REISINGER.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 694,658 MeurantMar. 4, 1902 2,080,520 Westbrook May 18, 1937 2,218,734 Westbrook Oct.22, 1940 2,233,500 Westbrook Mar. 4, 1941 2,406,072 Gaver Aug. 20, 19462,428,356 Chester et a1. Oct. 7, 1947 2 ,443,600 Chester June 22, 1948

